The Interplay of Near-Ambient O₃, Elevated CO₂, and Host Resistance Against Leaf Rust on Winter Wheat

This study investigated the effects of near-ambient ozone (O₃) and future predicted CO₂ concentrations on disease severity and progress of leaf rust (LR) on wheat caused by Puccinia triticina Eriks. Four winter wheat cultivars (Coker 9553, NC Neuse, Jamestown, and NuEast) with differential LR resistance were assessed for their O₃ responses to four O₃ treatments (subambient, 50, 75, and 100-ppb O₃) in continuously stirred tank reactors located in the greenhouse. O₃-induced foliar symptoms on the cultivars were either absent or negligible at a near-ambient O₃ concentration (50 ppb), but all cultivars showed visible injury symptoms at high O₃ concentrations. The effects of long-term near-ambient O₃ (50 ppb) and elevated CO₂ (570 ppm) on disease severity and disease components were also assessed on flag leaves after plants were inoculated with P. triticina race MBTNB at growth stage 39 to 40 Zadoks in outdoor plant environment chambers (OPECs). Infection was initiated by aerosol application of urediniospores after dew formation on leaves under high humidity conditions in the OPECs. Rust-resistant cultivar NuEast did not exhibit LR symptoms under gas treatments. Near-ambient O₃ singly or combined with elevated CO₂ (570 ppm) increased disease severity and pustule size and accelerated pustule formation on the susceptible cultivar Coker 9553. However, elevated CO₂ alone had no significant effect on disease severity. This study suggests that the interactive effect of greenhouse gases on wheat rust diseases could lead to enhanced rust epidemics.